Method and device for laser drilling organic materials

ABSTRACT

For the laser drilling of organic materials, in particular for making blind holes in dielectric layers, a frequency-doubled Nd-vanadate laser with the following parameters is used: 
     
       
         
               
               
               
               
             
                   
                   
               
                   
                 pulse width 
                 &lt;40 
                 ns 
               
                   
                 pulse frequency 
                 ≧20 
                 kHz 
               
                   
                 wavelength 
                 =532 
                 nm.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE00/03426 which has an Internationalfiling date of Sep. 29, 2000, which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention generally relates to laser drilling.

BACKGROUND OF THE INVENTION

It is known from EP-A-0 164 564 to use an excimer laser to produce blindholes in a laminate with the layer sequence metal-dielectric-metal. Theuppermost metal layer of the laminate is in this case used as anaperture mask, the pattern of holes of which is transferred by means ofphotolithography and is produced by subsequent etching. The dielectricexposed in the region of the apertures of this mask is then removed bythe action of the excimer laser until the lowermost metal layer isreached and the removal process is ended. The known method is used inparticular in the manufacture of multilayer printed circuit boards forproducing the required plated-through holes in the form of blind holes.

The German periodical “Feinwerktechnik & Messtechnik 91 (1983) 2, pages56-58, discloses a similar method of manufacturing multilayer printedcircuit boards, in which the blind holes serving as plated-through viasare produced with the aid of a CO₂ laser. Here, too, the uppermostcopper foil serves as an aperture mask, with which the copper is etchedaway whereever the laser beam is intended to produce a hole.

DE-A-197 19 700 also already discloses devices for the laser drilling oflaminates, in which a first laser with a wavelength in the range fromapproximately 266 nm to 1064 nm is used for drilling the metal layersand a second laser with a wavelength in the range from approximately1064 nm to 10600 nm is used for drilling the dielectric layers.

U.S. Pat. No. 5,593,606 discloses a method for the laser drilling oflaminates in which a single UV laser, the wavelength of which lies below400 nm and the pulse widths of which lie below 100 ns, is used fordrilling the metal layers and for drilling the dielectric layers.Precluding the use an excimer laser, metal and organic material areconsequently drilled with the same UV laser.

DE-A-198 24 225 discloses a further method for the laser drilling oflaminates, in which for example an SHG (second harmonic generation) YAGlaser with a wavelength of 532 nm or a THG (third harmonic generation)YAG laser with a wavelength of 355 nm can also be used for drilling themetal layers and for drilling the dielectric layers.

In principle, it can be stated that, in the laser drilling of organicmaterials with UV lasers, that is to say with wavelengths below 400 nm,a photochemical decomposition of the organic materials takes place.Consequently, no burning occurs and, on account of the extremely smallor non-existent thermal loading, in the case of laminates nodelamination occurs. By contrast with this, in the laser drilling oforganic materials with CO₂ lasers, a thermal decomposition of theorganic materials takes place, that is to say burning may occur and, inthe case of laminates, there is the risk of delamination. In comparisonwith UV lasers, however, considerably shorter machining times can beachieved with CO₂ lasers in the drilling of organic materials.

EP-A-0 478 313 discloses the so-called SLC (Surface Laminar Circuit)method, in which initially a first wiring level is produced on a basesubstrate. Then, a dielectric layer of a photosensitive epoxy resin isapplied to this first wiring level by screen printing or by curtaincoating. Then, blind holes are produced in the dielectric layer byphotolithographic means, by exposing and developing. After the chemicaland electrolytic copper-plating of the walls of the holes and thesurface of the dielectric layer, the second wiring level is produced bystructuring of the deposited copper layer. Further wiring levels can beproduced in the way described by the alternating application ofphotosensitive dielectric layers and copper layers.

SUMMARY OF THE INVENTION

An embodiment of the present invention is based on the problem ofpermitting rapid production of blind holes or through-holes withoutthermally damaging the material in the laser drilling of organicmaterials.

An embodiment of the invention is based on the finding that, withfrequency-doubled Nd-vanadate lasers with a wavelength of 532 nm andshort pulse widths of below 40 ns, layers of organic material can bemachined with short machining times and without the risk of burning. Inthis case, pulse frequencies of ≧20 kHz are chosen for the laserdrilling of the organic material. In the laser machining of the organicmaterials there is a combination of photochemical and thermaldecomposition, which in comparison with UV lasers permits shortermachining times and in comparison with CO₂ lasers avoids excessivethermal loads. It can be regarded as a further advantage that, with thesame Nd-vanadate laser, metal layers of laminates can also be drilled.For the drilling of metal layers of this type, pulse frequencies of ≧30kHz are then chosen.

The frequency-doubled ND-vanadate laser selected according to anembodiment of the invention for the drilling of organic materialspermits very high pulse frequencies, which may even lie above 100 kHz,with low pulse widths of less than 40 ns. The high pulse frequencies inthis case permit fast and effective machining of the organic materials,while very low thermal loading is ensured by the low pulse widths. Withother lasers which operate with similar or the same wavelengths, acombination of this type, with high pulse frequencies and short pulsewidths, cannot be realized. For example, in the case of the SHG-YAGlaser known from DE-A-198 24 225, at higher pulse frequencies it is onlypossible to achieve at most pulse widths of 70 to 80 ns.

An embodiment further can permit, by laser widths of less than 30 ns, astill lower thermal loading of the organic materials or, if appropriate,of the laminates during the laser drilling.

Using a focused laser beam with a spot diameter of between 10 μm and 100μm can provide effective laser machining of the organic materials. Usingspot diameters of between 20 μm and 50 μm can allow the laser machiningof the organic materials to be made even more effective.

An embodiment further can permit a considerable increase in themachining rate by the higher absorption of the laser beams in theorganic material. The additives are in this case to have a significantlyhigher degree of absorption for laser beams with a wavelength of 532 nmthan the pure organic material.

A development can permit a particularly simple and cost-effectiveincrease in the degree of absorption of the organic material.

A refinement can further permit an optimization of the degree ofabsorption by the selection of red additives, since the green light ofthe wavelength of 532 nm is absorbed particularly well by thecomplementary color red.

A development can specify a quantity range for the admixture of pigmentsas additive which has proven to be particularly successful forincreasing the degree of absorption without impairing the otherproperties. A narrower range can be regarded as optimum in this respect.

If the degree of absorption of the organic material is increased to atleast 50% by the admixture of additives a considerable increase in themachining rate in the organic material is already obtained. With anincrease in the degree of absorption to at least 60%, or to at least80%, the machining times for the laser drilling of the organic materialcan be reduced correspondingly further.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the examples described below, the following types of laser were used:

Laser I:

Diode-pumped, frequency-doubled Nd-vanadate laser from the companySpectra Physics, Mountain View, Calif., US.

Designation: T80-YHP40-532QW Wavelength: 532 nm Power: approximately 8.5W Operating mode: monomode TEMoo Pulse width: 20 ns at pulse frequencyof 10 kHz Pulse frequency: up to 200 kHz Field size: 100 × 100 mm².

Laser II:

Diode-pumped, frequency-doubled Nd-vanadate laser from the companyHaas-Laser GmbH, Schramberg, DE.

Designation: none, since prototype Wavelength: 532 nm Power:approximately 4.0 W Operating mode: monomode TEMoo Pulse width: 25 ns atpulse frequency of 10 kHz Pulse frequency: up to 200 kHz Field size: 100× 100 mm².

Example 1

In the manufacture of multilayer wirings, dielectric layers of anorganic material are applied in a thickness of, for example, 25 μm tothe already finished wiring layers by curtain coating or by screenprinting. An epoxy material is used, for example, as the organicmaterial. Then blind holes, later serving as plated-through vias to thenext wiring layers, are made in these dielectric layers without the useof masks.

For making blind holes in the dielectric layers, the laser II was used.Using two galvanometer mirrors for deflecting the laser beam, a surfacearea of 10 cm×10 cm can be machined. Further parameters of the laser arespecified as follows:

Pulse width: 30 ns Pulse frequency: 25 kHz

With a spot diameter of the focused laser beam of approximately 25 μm,the blind holes were made in the dielectric layer at the predeterminedlocations. A pulse frequency of between 10 and 20 kHz was chosen forthis. When making the blind holes, it was possible to avoid burning orother thermal damage.

Example 2

With the laser I, blind holes with a diameter of 125 μm were made in theepoxy material of an RCC material (RCC=Resin Coated Copper Foil). TheRCC material comprised a 12 μM thick copper foil and a 60 μm thickdielectric layer of epoxy material. The pulse frequency was 25 kHz. Thepulse length was 30 ns.

Using two galvanometer mirrors for deflecting the laser beam in the Xdirection and in the Y direction, a surface area of 10 cm×10 cm wasmachined. For drilling the epoxy material, the laser beam was set 1.6 mmout of focus (OOF=Out Of Focus) and moved in concentric circles in theregion of the hole. The linear velocity of the laser beam was 900 mm/s.After drilling through the epoxy material, the copper layer lyingunderneath was affected only slightly.

The drilling of the epoxy material took place at a rate of 220 holes persecond.

Example 3

As a departure from example 2, the laser II was used, with the samelaser parameters. The drilling of the epoxy material took place at arate of 122 holes per second.

Example 4

As a departure from example 2, the blind holes were made in a 60 μmthick FR4 material (FR4=level 4 fire retardant epoxy-glass composition),onto which a 12 μm thick copper foil had been laminated on one side. Theresults were comparable.

Example 5

As a departure from example 3, the blind holes were made in a 60 μmthick FR4 material, onto which a 12 μm thick copper foil had beenlaminated on one side. The results were comparable.

Example 6

As a departure from example 2, blind holes with a diameter of 100 μmwere produced. The drilling of the epoxy material took place here at arate of 382 holes per second.

Example 7

As a departure from example 3, blind holes with a diameter of 100 μmwere produced. The drilling of the epoxy material took place here at arate of 212 holes per second.

Example 8

As a departure from example 2, blind holes with a diameter of 75 μm wereproduced. The drilling of the epoxy material took place here at a rateof 800 holes per second.

Example 9

As a departure from example 3, blind holes with a diameter of 75 μm wereproduced. The drilling of the epoxy material took place here at a rateof 400 holes per second.

Example 10

As a departure from example 2, a modified epoxy material to whichapproximately 1.5% by weight of additive was admixed was used. Theadditive was an organic red pigment with the designation “1501 Fast Red”(C.I. Pigment Red 48:1) from the company Xijingming, Shenzhou City,Hebei Province, P.R. China. This pigment is an azo pigment based on abarium salt. The improved absorption of the laser radiation allowed therate for drilling the epoxy material to be increased to 550 holes persecond.

Example 11

As a departure from example 10, an inorganic red pigment with thedesignation “Bayferrox™” (C.I. Pigment Rot) from Bayer AG, DE was usedas the additive. This pigment is an iron oxide red pigment. The resultswere comparable.

Example 12

As a departure from example 10, a polymer-soluble anthraquinone dye withthe designation “Oracet™” Gelb GHS(C.I. Solvent Gelb 163) fromCIBA-GEIGY AG, CH was used as the additive. The increase in the rate fordrilling the epoxy material was somewhat smaller here.

Example 13

As a departure from example 10, the laser II was used, with the samelaser parameters. It was possible to increase the rate for drilling theepoxy material to 306 holes per second.

Example 14

As a departure from example 10, blind holes with a diameter of 100 μmwere produced. The rate for drilling the epoxy material was 956 holesper second.

Example 15

As a departure from example 13, blind holes with a diameter of 100 μmwere produced. The rate for drilling the epoxy material was 531 holesper second.

Example 16

As a departure from example 4, a modified FR4 material was used, inwhich, instead of the customary glass fiber reinforcement, the epoxymaterial was reinforced with approximately 50% by weight of fibers of aruby glass. This ruby glass was prepared by adding 2% by weight ofselenium, 1% by weight of arsenic trioxide and 0.5% by weight of carbonto a basic glass of the composition Na₂O—ZnO—4SIO₂.

It was possible to increase the rate for drilling this glass-reinforcedepoxy material by a factor of between 2 and 2.5.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are

1. A method for laser drilling of organic materials, comprising: using afrequency-doubled Nd-vanadate laser for the laser drilling, wherein thelaser includes the following laser parameters, pulse width <40 ns, pulsefrequency ≧20 kHz, and wavelength =532 nm.


2. The method as claimed in claim 1, wherein a laser pulse width of <30ns is used.
 3. The method as claimed in claim 1, wherein a focused laserbeam with a spot diameter of between 10 μm and 100 μm is used.
 4. Themethod as claimed in claim 3, wherein a focused laser beam with a spotdiameter of between 20 μm and 40 μm is used.
 5. The method as claimed inclaim 1, wherein additives which absorb laser beams with a wavelength of532 nm are admixed with the organic material.
 6. The method as claimedin claim 5, wherein at least one of an inorganic pigment, an organicpigment, at least one polymer-soluble dye and at least one fibrousfiller is used as the additive.
 7. The method as claimed in claim 6,wherein at least one of an inorganic red pigment and one organic redpigment and one polymer-soluble red dye is used as the additive.
 8. Themethod as claimed in claim 6, wherein between 0.1% by weight and 5.0% byweight of pigments are admixed with the organic material.
 9. The methodas claimed in claim 6, wherein between 1% by weight and 2% by weight ofpigments are admixed with the organic material.
 10. The method asclaimed in claim 5, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 50% forthe wavelength 532 nm of the laser radiation.
 11. The method as claimedin claim 5, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 60% forthe wavelength 532 nm of the laser radiation.
 12. The method as claimedin claim 5, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 80% forthe wavelength 532 nm of the laser radiation.
 13. A device for the laserdrilling of organic materials, comprising: a frequency-doubledNd-vanadate laser with the following laser parameters, pulse width <40ns, pulse frequency ≧20 kHz, and wavelength =532 nm.

.
 14. The method as claimed in claim 2, wherein a focused laser beamwith a spot diameter of between 10 μm and 100 μm is used.
 15. The methodas claimed in claim 14, wherein a focused laser beam with a spotdiameter of between 20 μm and 40 μm is used.
 16. The method as claimedin claim 7, wherein between 0.1% by weight and 5.0% by weight ofpigments are admixed with the organic material.
 17. The method asclaimed in claim 7, wherein between 1% by weight and 2% by weight ofpigments are admixed with the organic material.
 18. The method asclaimed in claim 6, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 50% forthe wavelength 532 nm of the laser radiation.
 19. The method as claimedin claim 7, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 50% forthe wavelength 532 nm of the laser radiation.
 20. The method as claimedin claim 8, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 50% forthe wavelength 532 nm of the laser radiation.
 21. The method as claimedin claim 9, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 50% forthe wavelength 532 nm of the laser radiation.
 22. The method as claimedin claim 6, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 60% forthe wavelength 532 nm of the laser radiation.
 23. The method as claimedin claim 7, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 60% forthe wavelength 532 mm of the laser radiation.
 24. The method as claimedin claim 8, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 60% forthe wavelength 532 nm of the laser radiation.
 25. The method as claimedin claim 9, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 60% forthe wavelength 532 nm of the laser radiation.
 26. The method as claimedin claim 6, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 80% forthe wavelength 532 nm of the laser radiation.
 27. The method as claimedin claim 7, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 80% forthe wavelength 532 nm of the laser radiation.
 28. The method as claimedin claim 8, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 80% forthe wavelength 532 nm of the laser radiation.
 29. The method as claimedin claim 9, wherein the organic material has, as a result of theadmixing of the additives, a degree of absorption of at least 80% forthe wavelength 532 nm of the laser radiation.
 30. A method for laserdrilling of metallic materials, comprising: using a frequency doubledNd-vanadate laser for the laser drilling, wherein the laser includes thefollowing laser parameters: pulse width < 40 ns, pulse frequency ≧ 30kHz, and wavelength = 532 nm.


31. The method as claimed in claim 30, wherein a laser pulse width ofless than 30 ns is used.
 32. The method as claimed in claim 30, whereina focused laser beam with a spot diameter of between 10 μm and 100 μm isused.
 33. The method as claimed in claim 32, wherein a focused laserbeam with a spot diameter with 20 μm and 40 μm is used.
 34. A device forthe laser drilling of metallic materials comprising: a frequency doubledNd-vanadate laser with the following laser parameters: pulse width < 40ns. pulse frequency ≧ 30 kHz, and wavelength = 532 nm.


35. The method as claimed in claim 34, wherein a focused laser beam witha spot diameter of between 10 μm and 100 μm is used.
 36. The method asclaimed in claim 35, wherein a focused laser beam with a spot diameterwith 20 μm and 40 μm is used.